No error when initializing a class field defined as a read-only getter in base class

[trusktr]

TypeScript Version: 3.6.4

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Code

class Base {
  get foo() {
    return 5
  }
}

class Child extends Base {
  foo = 10
}

new Child // runtime error!

Expected behavior:

Type error during compile

Actual behavior:

No error until runtime.

Playground Link:

playground

Related Issues:

Might be duplicate of #13347, but not sure.

[AviVahl]

Checked this snippet of code directly in Chrome, and there's no runtime error when evaluating new Child. I get a new Child instance with a foo field === 10, and it is assignable (unlike instances of new Base).
do you mean you get a runtime error if you transpile this using typescript (to es5) and then run?

I believe this issue has to do with: #27644

[fatcerberus]

I don't know how TypeScript is supposed to handle this, but classfields in ES use [[Define]] (think Object.defineProperty) which bypasses the setter in the base class, so this is legal JS; that's probably why you're not getting a runtime error in Chrome Avi Vahl (@AviVahl).

[trusktr]

Avi Vahl (@AviVahl) Yeah, exactly, if TS compiles to the form

constructor() {
  this.foo = 10
}

then it doesn't work. This can happen with either es5 target, or the new "useDefineForClassFields": false option (I haven't tested it with the latter).